Noise levels in our technological society are increasing to dangerous levels and present a real hazard to persons who are required to experience relatively high noise levels over a prolonged period of time. Scientists, doctors and government have become concerned about high noise level intensities in work environments which can ultimately result in a loss of hearing. In 1970, the Federal Government enacted the Occupational Safety and Health Act, having as part of its provisions sections designed to protect millions of industrial workers against the inevitable hearing loss due to the harmful noise level in their working environments.
Close proximity to jet aircraft engines, jackhammers, punch presses, and many industrial processes can subject a person to harmful noise levels of 80 dBA (decibels) and above. If prolonged exposure occurs, loss of hearing will result. For example, California legislation indicates that a person should not be exposed to a sound level of 110 dBA for a period of longer than one-half hour each day.
Many leisure activities may also expose a person to dangerous high sound levels. For example, using equipment such as motorcycles, chain saws, race cars, power lawn mowers and the like can subject an operator of such equipment to harmful noise levels. A snowmobile may have a noise level in the range of 105 dBA and racing motorcycles may reach as high as 110 dBA. Thus, in these situations, it is necessary to take some precautions to prevent loss of hearing. In fact, many pilots have lost their hearing in certain frequency ranges due to the constant and damaging noise levels emitted by piston-type aircraft engines. All sounds having excessively high noise levels of intensity can impair one's hearing, and this includes some of the popular music played on high amplifiers systems at high volumes. Obviously, depending both on the noise amplitude and the frequency, the impairment of one's hearing occurs in the low or high frequencies first. A total loss of hearing can result, depending on the period of exposure to such noise.
Since sound is propagated by pressure waves traveling through the air to one's ears, the hearing loss in many of the above situations can be prevented by wearing protective devices which either partially or totally attenuate these pressure waves. It is quite common for gun enthusiasts to wear ear plugs to attenuate the 140+ dBA impulse sound levels caused by firing a gun.
Regardless of the activity involved for personnel in work environments or in the pursuit of leisure time activities, they are often experiencing dangerously high sound levels and should wear some type of protective device. In fact, in industry, such devices are required by law, and vary from ear-inserted plugs of various kinds to a type of ear muff headgear that is worn over the head, much like ear phones.
The most common type of protective device to prevent loss of hearing in these situations is an ear-inserted plug. Some typical types of ear molds now marketed are shown in U.S. Pat. No. 3,565,069 and U.S. Pat. No. Re. 28,560. In these two patents, a bulbous portion of the ear-insertable mold is received into the auditory meatus (canal) of the ear to hold the mold in place. Obviously, if this bulbous portion does not properly fit the ear canal, sound can by-pass the plug and damage the tympanic membrane (ear drum), regardless of the attenuation provided by the sound-passing channel in the ear mold, if it has one.
With the proper fit, some ear plugs (without a sound passing channel) completely close off the ear canal and prevent any sound from reaching the ear drum except that which is transmitted through the surrounding bone structure. However in ear plugs, it is generally preferred to use devices which vent the ear canal to allow the pressures to equalize within the ear canal with ambient pressure and to pass non-injurious sound waves to the ear drum. This is very desirable since a wearer, where the ear canal is totally occluded by an ear plug, can experience symptoms of vertigo or dizziness, and further, plugs which totally occlude the ear canal are undesirable where a person is experiencing ambient pressure changes, such as when operating an aircraft. For this reason, protective ear devices having a small passage therethrough to allow air pressure equalization are the more preferred type. Also, it is not desirable to prevent a person from hearing sounds totally, since surrounding sounds provide a degree of warning and safety, especially to personnel on production lines. Thus, ideally, an ear-protecting device is one which will pass sound waves of a non-injurious level or intensity while blocking sound levels which can cause damage to the ear drum. U.S. Pat. No. Re. 28,560 attempts to provide such a feature using a membrane to close off the auditory channel in the ear plug when the sound levels passing therethrough exceed a safe level. U.S. Pat. No. 3,565,069 alternatively attempts to achieve the same feature by a resonant filter. Further, any device which reduces the cross-sectional area of the aperture of the ear canal will provide some attenuation of dangerous noise levels, since the sound pressure wave front reaching the ear drum is decreased due to the smaller cross-sectional area of the ear canal with the ear plug in place.
While in most situations it is desirable that the wearer of an ear-protective device be able to hear normal conversation so he can respond to warnings and other stimuli and also use the telephone, there are circumstances where the noise levels become so significant that total occlusion of sound waves in the ear canal is desirable. In view thereof, the most ideal device would be one which the wearer can convert from partial attenuation to total occlusion of sound waves, when the necessity arises. However, to achieve this end, it is still necessary that the portion of the ear-protective device which is inserted into the ear canal fit properly to prevent sound waves from bypassing the device.
The very best sound seal between a device and the ear canal can be obtained with a custom-fitted ear plug which contains a sound channel and sound attenuating device therein. Custom-fitted ear plugs are made by taking an actual impression of each individual ear canal and subsequently casting ear plugs having a perfect fit in the canal. In such a custom-fitted plug a complete sound seal between the ear canal and the device is obtained and the units tend to be quite comfortable when in use. Obviously, the difficulty of custom-fitted ear plugs is that they are costly to produce and can be used by only a single individual for whom they are made. This is true since each person's ear canals are as different as are his fingerprints.
An alternative to the custom-fitted ear plug is a universal device made in several sizes and having on a portion thereof inserted into the ear canal means for effecting a complete sound seal without creating discomfort. Obviously if the ear-protective device is ill-fitting, it will easily dislodge from the ear and become lost, and further, if such devices are uncomfortable, workers will resist wearing them or conveniently lose them in situations where they should be worn for their own protection.